BUG: Allow legacy dtypes to cast to datetime again

This constraint was added out of a caution with the thought that nobody
uses it.  Turns out ora does use it.
In practice, this only affects cast to and from datetimes (or possibly
strings but that seems even less likely).

Tested via ora (see gh-21365) the paths are not particularly special,
but the parametric dtype resolution is in principle missing.
This is not particularly problematic, since NumPy barely exposes that
though, and it never worked.

Closes gh-21365

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